Fixing apparatus

ABSTRACT

A fixing apparatus configured to include a rotational member having a conductive layer, a coil having a helical shape, a resonance circuit including a resonance capacitor and configured to be formed together with the rotational member and the coil, a first converter driving the resonance circuit, a second converter used to control power to be supplied to the first converter, a frequency setting unit configured to set a driving frequency of the first converter according to at least one of a size of the recording material and a temperature at a sheet non-passing portion of the rotational member, and a power control unit controlling the second converter according to a temperature at a sheet passing portion of the rotational member to control the power to be supplied to the first converter from the second converter, wherein the conductive layer is caused to generate heat by electromagnetic induction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fixing apparatus mounted on an imageforming apparatus, such as an electrophotographic copying machine,printer or the like.

2. Description of the Related Art

A fixing apparatus mounted on an image forming apparatus, such as anelectrophotographic copying machine, printer or the like, is generallyconfigured to heat a recording material bearing an unfixed toner imagewhile conveying the recording material at a nip portion formed by aheating rotational member and a pressing roller in contact with eachother, thereby fixing the toner image onto the recording material.

In recent years, a fixing apparatus employing the electromagneticinduction heating method, which can cause a conductive layer of theheating rotational member to generate heat, has been developed and putinto practical use. The fixing apparatus employing the electromagneticinduction heating method has such an advantage that the warm up time isshort.

Japanese Patent Application Laid-Open No. 2014-026267 discusses a fixingapparatus that can ease limitations imposed on a thickness and amaterial of a conductive layer.

Even the fixing apparatus discussed in Japanese Patent ApplicationLaid-Open No. 2014-026267 cannot be free from an issue of an increase ina temperature at a sheet non-passing portion when a small-sizedrecording material is processed by the fixing processing.

The present invention is directed to providing a fixing apparatuscapable of easily controlling a temperature of a region of therotational member that the recording material passes through whilecreating a heat generation distribution according to a size of therecording material.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a fixing apparatusconfigured to fix an image onto a recording medium includes a rotationalmember having a cylindrical shape and configured to include a conductivelayer, a coil having a helical shape and configured to be disposedinside the rotational member, the coil having a helix axis extending ina direction along a generatrix direction of the rotational member, aresonance circuit including a resonance capacitor and configured to beformed together with the rotational member and the coil, a firstconverter configured to drive the resonance circuit, a second converterconfigured to be used to control power to be supplied to the firstconverter, a frequency setting unit configured to set a drivingfrequency of the first converter according to at least one of a size ofthe recording material and a temperature at a sheet non-passing portionof the rotational member, and a power control unit configured to controlthe second converter according to a temperature at a sheet passingportion of the rotational member to control the power to be supplied tothe first converter from the second converter, wherein the conductivelayer is caused to generate heat by electromagnetic induction, and theimage formed on the recording material is fixed onto the recordingmaterial with the heat of the rotational member.

According to another aspect of the present invention, a fixing apparatusconfigured to fix an image onto a recording medium includes a rotationalmember having a cylindrical shape and configured to include a conductivelayer, a coil having a helical shape and configured to be disposedinside the rotational member, the coil having a helix axis extending ina direction along a generatrix direction of the rotational member, aresonance circuit including a resonance capacitor and configured to beformed together with the rotational member and the coil, a firstconverter configured to drive the resonance circuit, a second converterconfigured to be used to control power to be supplied to the firstconverter, a frequency setting unit configured to set a drivingfrequency of the first converter according to at least one of a size ofthe recording material and a temperature at a sheet non-passing portionof the rotational member, and a power control unit configured to controlthe second converter according to a temperature at a sheet passingportion of the rotational member and the driving frequency to controlthe power to be supplied to the first converter from the secondconverter, wherein the conductive layer is caused to generate heat byelectromagnetic induction, and the image formed on the recordingmaterial is fixed onto the recording material with the heat of therotational member.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an image formingapparatus.

FIG. 2 is a cross-sectional view illustrating a fixing unit.

FIG. 3 is a front view illustrating the fixing unit.

FIG. 4 is a perspective view illustrating a coil unit mounted on thefixing unit and a block diagram of a driving circuit.

FIG. 5 is a diagram illustrating the driving circuit.

FIG. 6 is a diagram illustrating a relationship between a drivingfrequency and a temperature distribution of a fixing sleeve.

FIG. 7 is a diagram illustrating an operation of a first converter.

FIG. 8 is a diagram illustrating an operation of the first converterwhen the driving frequency is switched.

FIG. 9 is a diagram illustrating an operation of a second converter.

FIG. 10 is a diagram illustrating an operation of the second converterwhen an ON duty ratio of a switching element in the second converter isswitched.

FIG. 11 is a diagram illustrating a difference in a heat generationamount of a rotational member when the ON duty ratio of the switchingelement in the second converter is switched.

FIG. 12 is a diagram illustrating a relationship between a drivingfrequency and a temperature distribution of a fixing sleeve according toa second exemplary embodiment.

FIG. 13 is a diagram illustrating a relationship between an ON dutyratio and an input power duty ratio.

FIGS. 14A, 14B, and 14C are diagrams illustrating a relationship betweenthe driving frequency and input power, and how the input power dutyratio is changed.

FIG. 15 is a flowchart illustrating a processing procedure according toa first exemplary embodiment.

FIG. 16 is a flowchart illustrating a processing procedure according toan exemplary modification of the first exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

In the following description, how the present invention can be embodiedwill be described in detail with reference to the drawings based onexemplary embodiments by way of example. However, dimensions, materials,shapes, a relative layout, and the like of component parts that will bedescribed in these exemplary embodiments should be changed as necessaryaccording to a configuration of an apparatus to which the presentinvention is applied, and various kinds of conditions. In other words,the present disclosure is not intended to limit the scope of the presentinvention to the exemplary embodiments that will be described below.

FIG. 1 is a schematic configuration diagram illustrating an imageforming apparatus 100 according to a first exemplary embodiment of thepresent invention. The image forming apparatus 100 according to thepresent exemplary embodiment is a laser beam printer using theelectrophotographic process.

A controller 31 is a control unit of the image forming apparatus 100 andincludes a central processing unit (CPU) (a central processing device)32, various kinds of input and output control circuits (notillustrated), and the like. The CPU 32 includes a read only memory (ROM)32 a, a random access memory (RAM) 32 b, a timer 32 c, and the like. Anelectrophotographic photosensitive member 101 configured as a rotationaldrum (hereinafter referred to as a photosensitive drum) serves as animage bearing member, and is rotationally driven at a predeterminedcircumferential speed in a clockwise direction indicated by an arrow.The photosensitive drum 101 is uniformly charged by a contact chargingroller 102 during the rotation process thereof so as to have apredetermined polarity and a predetermined potential. A laser beamscanner 103 outputs a laser light L on-off modulated according to imageinformation input from a not-illustrated external apparatus, such as animage scanner and a computer. A charged surface of the photosensitivedrum 101 is exposed by the laser light L, and an electrostatic latentimage corresponding to the image information is formed on the surface ofthe photosensitive drum 101. A developing device 104 supplies adeveloper (toner) from a developing roller 104 a onto the surface of thephotosensitive drum 101, thereby developing the electrostatic latentimage formed on the surface of the photosensitive drum 101 as a tonerimage. A sheet feeding cassette 105 contains recording materials P. Aregistration roller 107 conveys a recording material P in such a mannerthat a leading edge of the toner image formed on the photosensitive drum101 and a predetermined position of the recording material P coincidewith each other. Upon an input of a sheet feeding start signal, a sheetfeeding roller 106 is driven, and the recording materials P contained inthe sheet feeding cassette 105 are fed one by one. A fed recordingmaterial P is introduced into a transfer portion 108T, where thephotosensitive drum 101 and a transfer roller 108 are in abutment witheach other, after a conveyance timing is adjusted by the registrationroller 107. While the recording material P is being held and conveyed atthe transfer portion 108T, a transfer bias is applied from anot-illustrated power source to the transfer roller 108. The transferbias having an opposite polarity from a charged polarity of the toner isapplied to the transfer roller 108, by which the toner image formed onthe photosensitive drum 101 is transferred onto the recording materialP. After that, the recording material P with the toner image transferredthereon is separated from the surface of the photosensitive drum 101,and is introduced into a fixing unit A after passing through aconveyance guide 109. The toner image formed on the recording material Pis heated and fixed onto the recording material P at the fixing unit A.After passing through the fixing unit A, the recording material P isdischarged onto a sheet output tray 112 via a sheet output port 111.Meanwhile, the surface of the photosensitive drum 101 after therecording material P is separated therefrom is cleaned at a cleaningportion 110.

The fixing unit A is a fixing apparatus that operates based on theelectromagnetic induction heating method. More specifically, the fixingunit A is a fixing apparatus that causes a conductive layer of arotational member to generate heat by electromagnetic induction with useof a magnetic flux generated by a coil, and fixes the image formed onthe recording material P onto the recording material P by the heat ofthe rotational member. FIG. 2 is a cross-sectional view illustrating thefixing unit A. FIG. 3 is a front view illustrating the fixing unit A.FIG. 4 is a perspective view illustrating a coil unit mounted on thefixing unit A. The fixing unit A includes a heating unit having a fixingsleeve 1 and a coil unit, which will be described below, and a pressingmember 8, and forms a fixing nip portion N, where the recording materialP bearing the unfixed toner image is conveyed while being held betweenthe heating unit and the pressing member 8.

The pressing roller 8 as the pressing member includes a core metal 8 a,an elastic layer 8 b made of silicone rubber or the like, and a releaselayer 8 c made of fluorine-contained resin or the like. Both ends of thecore metal 8 a are rotatably held between not-illustrated apparatuschassis of the fixing unit A via bearings. Further, each of pressingsprings (compression springs in the present exemplary embodiment) 17 aand 17 b is disposed at a position between a different end among bothends of a pressing stay (a metallic reinforcing member) 5 and acorresponding spring bearing member (i.e., a spring bearing member 18 aor 18 b) on the apparatus chassis side illustrated in FIG. 3, by which apush-down force is applied to the pressing stay 5. At the fixing unit Aaccording to the present exemplary embodiment, a pressing force ofapproximately 100 N to 250 N in total (approximately 10 kgf toapproximately 25 kgf) is applied to the pressing stay 5. By thisconfiguration, a bottom surface of a sleeve guide member 6 made ofthermally-resistant resin (for example, Polyphenylenesulfide (PPS)) andthe pressing roller 8 are in pressure contact with each other via thefixing sleeve 1, thereby forming the fixing nip portion N. The pressingroller 8 is driven by a not-illustrated driving unit in a directionindicated by an arrow, and the fixing sleeve 1 rotates by being drivenby the rotation of the pressing roller 8. Flange members 12 a and 12 brotate by being driven by the rotation of the fixing sleeve 1. Theflange members 12 a and 12 b are rotatably disposed at longitudinal endsof the sleeve guide member 6. When the fixing sleeve 1 is displacedtoward one side in a generatrix direction during the rotation, thefixing sleeve 1 abuts against the flange member 12 a or 12 b, and theflange member 12 a or 12 b pushed by the fixing sleeve 1 abuts against aregulating member 13 a (or 13 b). As a result, the one-sideddisplacement of the fixing sleeve 1 is regulated by the regulatingmember 13 a or 13 b. The flange members 12 a and 12 b are made of ahighly thermally-resistant material, such as a liquid crystal polymer(LCP).

The fixing sleeve 1 as a rotatable cylindrical rotational memberdesirably have a diameter of 10 to 50 mm. The fixing sleeve 1 includes aheat generation layer (a conductive layer) 1 a serving as a base layer,an elastic layer 1 b layered on an outer surface of the heat generationlayer 1 a, and a release layer 1 c as a front surface of the fixingsleeve 1. The heat generation layer 1 a is a metallic film (a stainlessmaterial for the fixing sleeve 1 in the present exemplary embodiment),and desirably have a film thickness of 10 to 50 μm. The elastic layer 1b is made of silicone rubber, and desirably have a hardness ofapproximately 20 degrees (Japanese Industrial Standards (JIS)-A, under aweight of one kg) and a thickness of 0.1 to 0.3 mm. The release layer 1c is a fluorine-contained resin tube, and desirably have a thickness of10 to 50 μm. An induced current is generated in the heat generationlayer 1 a from an effect of an alternating magnetic flux, which will bedescribed below. The heat generation layer 1 a generates heat by theinduced current, and the heat is transmitted to the elastic layer 1 band the release layer 1 c, whereby the fixing sleeve 1 is heatedentirely in a circumferential direction. Temperature detection elements9, 10, and 11, which detect temperatures of the fixing sleeve 1, will bedescribed below.

A mechanism for generating the induced current in the heat generationlayer 1 a will be described in detail. FIG. 4 is the perspective viewillustrating the coil unit mounted on the heating unit. The coil unitincludes a coil 3. The coil 3 includes a helical shaped portion disposedinside the rotational member (the fixing sleeve) 1, and having a helixaxis extending substantially in parallel with the generatrix directionof the rotational member 1. The coil 3 generates an alternating magneticfield for causing the conductive layer 1 a of the rotational member 1 togenerate the heat by the magnetic induction. Further, the coil unitincludes a core 2 disposed inside the helical shaped portion and used toguide the magnetic flux. The core 2 as a magnetic core material isdisposed so as to penetrate through a hollow portion of the fixingsleeve 1 with use of a not-illustrated fixation unit. The core 2 hasmagnetic poles of North Pole (NP) and South Pole (SP). The core 2 isshaped so as to form no loop outside the rotational member 1 (i.e., ashape having an end), and the magnetic flux generated by the coil 3forms an open magnetic path. The core 2 is desirably formed of a highmagnetic permeability material made of a material having a smallhysteresis loss and a high relative magnetic permeability, for example,a ferromagnetic oxidized material or alloy including a calcined ferrite,ferrite resin, an amorphous alloy, a permalloy, or the like. Accordingto the present exemplary embodiment, a calcined ferrite having arelative magnetic permeability of 1800 is used for the core 2. The core2 according to the present exemplary embodiment is cylindrically shaped,and desirably have a diameter of 5 to 30 mm. In a case where the fixingunit A is a fixing apparatus mounted on an A4 printer, a length of thecore 2 is desirably approximately 240 mm. The core 2 with the coil 3wound around it is covered with a resin cover 4.

The energizing coil 3 is formed by placing a single conductive wire inthe hollow portion of the fixing sleeve 1, and helically winding theconductive wire around the core 2. The conductive wire is wound in sucha manner that an interval is shorter at ends of the core 2 than at acentral portion of the core 2. In the case where the core 2 has thelongitudinal dimension of 240 mm, the energizing coil 3 is woundeighteen times around the core 2. The interval between the turns thereofis 10 mm at the ends, 20 mm at the central portion, and 15 mm atintermediate portions therebetween. In this manner, the coil 3 is woundin a direction intersecting with an axis X of the core 2.

When a high-frequency current is applied from a high-frequency converterto the energizing coil 3 via power supply contact portions 3 a and 3 b,magnetic fluxes are generated. The apparatus according to the presentexemplary embodiment is designed in such a manner that most of magneticfluxes exiting from the end of the core 2 (70% or more, desirably, 90%or more, and further desirably, 94% or more) pass through outside theheat generation layer 1 a of the fixing sleeve 1 to return to theopposite end of the core 2. Therefore, the induced current flowing inthe circumferential direction is generated in the heat generation layer1 a of the fixing sleeve 1 in order that a magnetic flux that cancelsout the magnetic flux passing through outside the sleeve 1 is generated.As a result, the heat generation layer 1 a generates the heat entirelyin the circumferential direction. In this manner, in the case where thefixing apparatus is configured to cause the induced current to flow inthe circumferential direction of the fixing sleeve 1, the fixing sleeve1 generates the heat over the entire region in the circumferentialdirection thereof, whereby this configuration has a merit of allowingthe fixing apparatus to warm up to a fixable temperature in a reducedtime period. Further, the core 2 has the shape having the ends, and isconfigured in such a manner that most of the magnetic fluxes passthrough outside the heat generation layer 1 a due to the open magneticpath. Therefore, the present exemplary embodiment also has such a meritthat the fixing apparatus can be reduced in size compared to anapparatus including a loop-shaped core and configured to form a closemagnetic path.

As illustrated in FIG. 2, the temperature detection elements 9, 10, and11 of the fixing unit A are disposed upstream than the fixing nipportion N in a rotational direction of the fixing sleeve 1, and detectthe temperatures of the surface of the fixing sleeve 1. Further, asillustrated in FIG. 3, the temperature detection elements 9, 10, and 11detect temperatures at a center and both ends of the fixing sleeve 1 ina longitudinal direction of the fixing unit A, respectively. Thetemperature detection elements 9, 10, and 11 are each embodied by athermistor or the like. Power supply to the coil 3 is controlled in sucha manner that the temperature detected by the temperature detectionelement 9 at the central portion is maintained at a control targettemperature suitable for fixing. Further, each of the temperaturedetection elements 10 and 11 disposed in the vicinity of the end of thefixing sleeve 1 can detect how much the temperature increases at a sheetnon-passing portion of the fixing sleeve 1 when images are successivelyprinted on a small-sized recording material P. Each of the temperaturedetection elements 10 and 11 may be disposed at the corresponding axialend of the pressing roller 8, and detect how much a temperatureincreases at a sheet non-passing portion of the pressing roller 8 whenthe images are successively printed on the small-sized recordingmaterial P.

FIG. 4 also includes a block diagram illustrating a relationship amongthe CPU 32, which is the control unit that controls the printer, aprinter controller 41, and a host computer 42. The printer controller 41performs communication with and receives image data from the hostcomputer 42, and rasterizes the received image data into informationthat the image forming apparatus 100 can print. Further, the printercontroller 41 exchanges a signal and performs serial communication withan engine control unit 121. The engine control unit 121 exchanges thesignal with the printer controller 41, and further controls each of theunits of the image forming apparatus 100 via serial communication. Theengine control unit 121, for example, controls the temperature of thefixing unit A based on the temperatures detected by the temperaturedetection elements 9, 10, and 11, and also detects an abnormality in thefixing unit A.

Meanwhile, it has been found out that the following issue arises withthe fixing apparatus designed in such a manner that most of the magneticfluxes exiting from the end of the core pass through outside the heatgeneration layer of the fixing sleeve to return to the opposite end ofthe core so as to generate the induced current flowing in thecircumferential direction of the sleeve in the conductive layer of thesleeve.

Generally, the fixing apparatus that operates based on theelectromagnetic induction method is provided with the high-frequencyconverter that drives a resonance circuit including a coil. Then, in thecase where the fixing apparatus employs the method for generating theheat by linking the magnetic flux generated by the coil to theconductive layer of the rotational member and generating an eddy currentin the conductive layer, the fixing apparatus adjusts a drivingfrequency of the high-frequency converter to adjust a heat generationamount so as to keep the temperature of the sleeve constant.

However, it has been found out that the fixing apparatus that generatesthe induced current flowing in the circumferential direction of thesleeve, according to the present exemplary embodiment, is subject to achange in a heat generation distribution of the sleeve in the generatrixdirection when the driving frequency of the high-frequency converter ischanged. FIG. 6 is a diagram illustrating temperature distributions ofthe sleeve when the driving frequency of the high-frequency converter ischanged within a range of 20 kHz to 50 kHz so as to maintain thetemperature at the center of the rotational member (the sleeve) in thegeneratrix direction (the longitudinal direction) at 200° C. This graphreveals that the heat generation amount reduces at the both ends of thesleeve as the driving frequency reduces. Therefore, for example, in acase where the driving frequency should be set to 20 kHz to maintain thetemperature at the central portion at 200° C., the heat generationamount falls short at the both ends of the sleeve. As a result, thefixing apparatus fails to completely fix the image on the recordingmaterial corresponding to the both ends of the sleeve.

Therefore, according to the present exemplary embodiment, in addition toa high-frequency converter 16 (a first converter) that drives aresonance circuit 191, a second converter 15 is included for controllingpower to be supplied to the first converter 16, as illustrated in FIGS.4 and 5. The resonance circuit 191 includes the cylindrical rotationalmember 1 having the conductive layer 1 a, the coil 3 disposed inside therotational member 1 and having the helix axis extending substantially inparallel with the generatrix direction of the rotational member 1, and aresonance capacitor 1113. It is sufficient that the helix axis of thecoil 3 extends along the generatrix direction of the rotational member1. The resonance circuit 191 illustrated in FIG. 5 has an equivalentresistance R of the fixing unit A and an equivalent inductance L of thefixing unit A. The resonance circuit 191 according to the presentexemplary embodiment is a current resonance circuit. Further, there isprovided a frequency setting unit 120 that sets the driving frequency ofthe first converter 16 according to at least one of a size of therecording material P and the temperature at the sheet non-passingportion of the rotational member 1. Furthermore, there is provided apower control unit 119 that controls the second converter 15 accordingto a temperature at a sheet passing portion of the rotational member 1to control the power to be supplied from the second converter 15 to thefirst converter 16. Each of the first converter 16 and the secondconverter 15 is an inverter that converts a direct current into analternating current, as narrowly defined.

More specifically, the frequency setting unit 120 sets the drivingfrequency of the first converter 16 according to the temperaturedetected by the temperature detection element 10 or 11 so as to preventan excessive increase in the temperature of the rotational member 1 on aregion that is the sheet non-passing portion where a small-sized sheetdoes not pass through. The power control unit 119 controls outputvoltage of the second converter 15 so as to maintain the temperature atthe sheet passing portion of the rotational member 1 (the temperaturedetected by the temperature detection element 9) at a control targettemperature, which is the fixable temperature. The driving frequency ofthe first converter 16 may be set according to information about thesize of the recording material P.

The driving circuit illustrated in FIG. 5 will be described in detail. Acommercial power source (an alternating-current power source) 50 isconnected to the image forming apparatus 100, and suppliesalternating-current power to the image forming apparatus 100. A waveformof the commercial power source 50 is a waveform shown as a waveform 1,where a horizontal axis and a vertical axis represent a time and avoltage, respectively. The power input from the commercial power source50 is input into a diode bridge 1102 via an alternating-current (AC)filter 1101, and is subject to full-wave rectification. After beingcharged in a capacitor 1103, the rectified voltage exhibits a voltagewaveform shown as a waveform 2, where a horizontal axis and a verticalaxis represent a time and a voltage, respectively.

A power source unit 71 generates a direct-current voltage, and outputs apredetermined voltage to a not-illustrated secondary-side load (a motor,the CPU, and the like).

The first converter 16 will be described. As will be described below,the first converter 16 is connected to an output of the second converter15. Switching elements 1108 and 1109 form a half-bridge circuit of thefirst converter 16. A capacitor 1110 is a voltage resonance capacitorand is connected to between a drain (D) and a source (S) of theswitching element 1109 (between a collector and an emitter, if theswitching element 1109 is an insulated gate bipolar transistor (IGBT)),according to the present exemplary embodiment. A switching elementdriving circuit 1118 drives the switching elements 1108 and 1109. Theresonance circuit 191 is a series resonance (current resonance) circuithaving the equivalent inductance L, the equivalent resistance R, andincluding the current resonance capacitor 1113. The equivalentresistance R corresponds to a resistance of the rotational member 1 anda resistance of the energizing coil 3 that are expressed as a seriesequivalent resistance from the point of view of the energizing coil 3.

FIG. 7 is a diagram illustrating a gate (G)-source (S) voltage of theswitching element 1108, a gate (G)-source (S) voltage of the switchingelement 1109, a drain (D) voltage of the switching element 1109, acurrent of the coil 3, and a voltage of the capacitor 1113. Because ofthe use of the current resonance circuit, both the switching elements1108 and 1109 are alternately driven at a duty ratio of approximately50% in terms of a time period 1+a time period 2+a time period 3+a timeperiod 4. More specifically, a time period during which the switchingelement 1108 is turned on is the time period 1, and a ratio of the timeperiod 1 is (the time period 1/(the time period 1+the time period 2+thetime period 3+the time period 4)) 50%. A time period during which theswitching element 1109 is turned on is the time period 3, and a ratio ofthe time period 3 is (the time period 3/(the time period 1+the timeperiod 2+the time period 3+the time period 4))≈50%. The switchingelements 1108 and 1109 are driven at the duty ratio of 50%, because halfthe voltage input into the first converter 16 should be charged in thecurrent resonance capacitor 1113. In a case where the switching elements1108 and 1109 are not driven at the duty ratio of 50%, this results in areduction in a voltage amplitude allowable in the current resonancecapacitor 1113, and thus the power that can be output to the coil 3 isreduced. Further, a dead time is necessarily provided as a time periodduring which the switching elements 1180 and 1109 are turned off at thesame time (the time period 2 and the time period 4 illustrated in FIG.7) to prevent both the switching elements 1108 and 1109 from beingconductive.

The capacitor 1110 is connected to between the drain (D) terminal andthe source (S) terminal of the switching element 1109. When theswitching element 1108 is turned on and the current flows from thecapacitor 1107, a voltage of the capacitor 1110 becomes substantiallyequal to a voltage of the capacitor 1107. After that, the current startsflowing in the energizing coil 3 and the capacitor 1113 in the fixingunit A (the time period 1 illustrated in FIG. 7). The current flowing inthe coil 3 and the capacitor 1113 has a sinusoidal waveform. Theswitching element 1108 is turned off while the current from the coil 3is charging the capacitor 1113. Because the current is kept urged toflow in the energizing coil 3 continuously, the current flows in thecapacitor 1113 and a not-illustrated reverse conducting diode includedin the switching element 1109 (the time period 2 illustrated in FIG. 7).

The drain (D) voltage of the switching element 1109 becomes lower than asource (S) voltage of the switching element 1109 by a degreecorresponding to a forward voltage of the reverse conducting diode. Thefrequency setting unit 120 turns on the switching element 1109 via theswitching element driving circuit 1118 while the reverse conductingdiode of the switching element 1109 is conducting the current during thetime period 2 illustrated in FIG. 7. The current flowing in theenergizing coil 3 reduces over time. The voltage stored in the capacitor1113 is maximized, and the current starts flowing in a reverse directionafter that (the time period 3 illustrated in FIG. 7).

The switching element 1109 is turned off before the current flowing inthe reverse direction reaches 0 A. Then, the flowing current startscharging the capacitor 1110, and the drain (D) voltage of the switchingelement 1109 increases (the time period 4 illustrated in FIG. 7). Whenthe drain (D) voltage of the switching element 1109 becomes higher thanthe voltage of the capacitor 1107, the current starts flowing in anot-illustrated reverse conducting diode included in the switchingelement 1108.

The voltage of the capacitor 1110 is a sum of the voltage of thecapacitor 1107 and the forward voltage of the not-illustrated reverseconducting diode included in the switching element 1109. The frequencysetting unit 120 turns on the switching element 1108 via the switchingelement driving circuit 1118 while the current is flowing in the reverseconducting diode of the switching element 1108 (the time period 1illustrated in FIG. 7). After that, the frequency setting unit 120repeats the above-described switching control from the time period 1 tothe time period 4.

In this manner, the switching elements 1108 and 1109 achieve a softswitching operation, whereby high efficiency can be maintained, byappropriate settings of a capacity of the voltage resonance capacitor1110, the current when the switching element 1108 or 1109 is turned off,and the time period of the dead time (the time period 2 and the timeperiod 4).

A switching frequency (the driving frequency) of the current resonancecircuit 191 is controlled by the frequency setting unit 120. Thefrequency setting unit 120 controls the driving frequency of theresonance circuit 191 based on the temperature detected by thetemperature detection element 10 or 11 disposed on the region of therotational member 1 where the recording material P does not pass through(the sheet non-passing portion). The sheet non-passing portion means aregion where a recording material having a largest size usable in theapparatus passes through but a recording material having a smaller sizethan the largest size does not pass through. For example, when thetemperature detected by the temperature detection element 10 or 11reaches a predetermined upper limit temperature, the driving frequencyof the resonance circuit 191 is reduced, so that the heat generation atthe sheet non-passing portion of the rotational member 1 is reduced tolimit the temperature increase at the sheet non-passing portion. In thismanner, the heat generation distribution suitable for the size of arecording material is established. FIG. 8 is a diagram illustratingwaveforms of the gate (G)-source (S) voltages of the switching elements1108 and 1109 when the driving frequency is set to kHz and 50 kHz. TheON duty ratio of the switching element 1108 and the ON duty ratio of theswitching element 1109 are approximately 50% for both of the drivingfrequencies. By switching the driving frequency of the first converter16 in this manner, the heat generation distribution suitable for thesize of a recording material can be established, like the heatgeneration distribution illustrated in FIG. 6. According to the presentexemplary embodiment, the driving frequency is controlled in such amanner that the temperatures detected by the temperature detectionelements 10 and 11 do not exceed the upper limit temperature, wherebythe driving frequency may be changed while a single recording materialis being processed by the fixing processing. On the other hand, in acase where the temperature detection elements 10 and 11 are notprovided, and the driving frequency of the resonance circuit 191 is setaccording to the information about the size of a recording material, theabove-described effect can be achieved by setting a predetermineddriving frequency for each size of a recording material. Thisconfiguration will be described in a second exemplary embodiment.

An operation of the second converter 15 will be described. The secondconverter 15 is provided to control the power to be supplied to thefirst converter 16, and controls the power to be supplied to the firstconverter 16 according to the temperature at the sheet passing portionof the rotational member 1 where the recording material P passes through(the temperature detected by the temperature detection element 9)regardless of the size of the recording material P. More specifically,the power control unit 119 transmits a signal to the driving circuit1117 according to the temperature detected by the temperature detectionelement 9 to control an ON duty ratio of a switching element 1104. As aresult, the power to be supplied to the first converter 16 (the outputvoltage of the second converter 15) is controlled.

The second converter 15 includes the switching element 1104, a diode1105, a coil 1106, the capacitor 1107, and the like, and is a voltagestep-down converter. A voltage is applied to between a gate (G) and asource (S) of the switching element 1104, and the voltage is applied tothe coil 1106 when the switching element 1104 is turned on. A differencevoltage between voltages of the capacitor 1103 and the capacitor 1107 isapplied to both ends of the coil 1106. A slope of a current flowing inthe coil 1106 is determined by an inductance of the coil 1106 and thevoltage applied to the coil 1106.

The current passed through the coil 1106 charges the capacitor 1107. Asthe voltage of the capacitor 1107 increases, the voltage applied to thecoil 1106 reduces even when the switching element 1104 is turned on. Inthis manner, the current flowing in the coil 1106 is changed accordingto the voltage applied to the coil 1106, but the current flowing in thecoil 1106 approximately linearly increases if the voltage of thecapacitor 1107 increases slowly. This time period is a time period 1illustrated in FIG. 9.

Turning off the switching element 1104 brings about such a state thatthe current continuously flows to the coil 1106 via the diode 1105. Thecapacitor 1107 is charged by power that is stored in the coil 1106 inthe form of a magnetic field. If a capacity of the capacitor 1107 issufficiently large, the current of the coil 1106 reduces with asubstantially linear characteristic line. If the current is flowing inthe coil 1106 when the switching element 1104 is turned on, a currentvalue at the time is an initial value when the switching element 1104 isturned on. The second converter 15 functions by repeating theabove-described series of operations.

Pulse width modulation (PWM) control is used for a method for drivingthe switching element 1104. The power control unit 119 increases an ONtime ratio of the PWM, i.e., the ON duty ratio (the time period 1/(thetime period 1+a time period 2) illustrated in FIG. 9), when it isdesired to increase the output power of the second converter 15 so as tomaintain the temperature at the sheet passing portion of the rotationalmember 1 at the control target temperature. Conversely, the powercontrol unit 119 reduces the ON duty ratio when it is desired to reducethe output power of the second converter 15.

In the PWM control, the current flowing in the coil 1106 never falls tozero. FIG. 9 is a diagram illustrating the current of the coil 1106 anda voltage of a K terminal of the diode 1105 when the PWM control isperformed. In this manner, the switching element 1104 functions as hardswitching, in which a turn-on operation and a turn-off operation areperformed while the current is flowing. The voltage of the capacitor1107 illustrated in FIG. 9 corresponds to the output voltage of thesecond converter 15.

FIG. 10 is a diagram illustrating a comparison between when the ON dutyratio is set to 80% and when the ON duty ratio is set to 50%. Asillustrated in FIG. 10, a change in the ON duty ratio leads to a changein the voltage of the capacitor 1107, and thus the output voltage of thesecond converter 15 is changed. This results in a change in the powersupplied to the first converter 16.

A noise may be created depending on a timing at which the switchingelement 1104 is turned on and off. In such a case, a critical mode, inwhich the switching element 1104 is kept turned off until the currentflowing in the coil 1106 reaches 0 A while the switching element 1104 isturned off, may be used.

Because the source (S) terminal of the switching element 1104 is acontact point between the coil 1106 and the diode 1105, the voltage herebecomes equivalent to a voltage of a negative-side terminal of thecapacitor 1103 when the switching element 1104 is turned off. Thisvoltage becomes equivalent to a voltage of a positive-side terminal ofthe capacitor 1103 when the switching element 1104 is turned on. In thismanner, the switching element 1104 is subject to a large change in thesource (S) voltage, which necessitates driving by transformer couplingor use of a not-illustrated bootstrap circuit to maintain continuoussupply of the voltage to between the gate (G) and the source (S) of theswitching element 1104.

The switching element 1104 is connected to the commercialalternating-current power source 50 without being insulated therefrom.The present exemplary embodiment is configured to secure insulation bythe driving circuits 1117 and 1118 by way of example, to allow thisconfiguration to be applied to an apparatus requiring insulation incompliance with a safety standard.

In the manner as described above, the power control unit 119 controlsthe ON duty ratio of the above-described PWM control based on thetemperature detected by the temperature detection element 9.Proportional-Integral (PI) control, Proportional-Integral-Derivative(PID) control, or the like is used as a control method therefor. Then,the power control unit 119 drives the driving circuit 1117 to controlthe ON duty ratio of the switching element 1104 so as to maintain thetemperature detected by the temperature detection element 9 at thecontrol target temperature, which is the fixable temperature. The changein the ON duty ratio of the switching element 1104 leads to the changein the voltage of the capacitor 1107, and thus the power supplied to thefirst converter 16 is changed.

According to the present exemplary embodiment, the output voltage of thesecond converter 15 is controlled, instead of controlling the drivingfrequency of the first converter 16, to maintain the temperature at thesheet passing portion (the temperature detected by the temperaturedetection element 9) at the control target temperature.

FIG. 11 is a diagram illustrating a comparison between the heatgeneration amount of the rotational member 1 when the ON duty ratio ofthe switching element 1104 of the second converter 15 is set to 80%, andthe heat generation amount of the rotational member 1 when this ON dutyratio is set to 50%. As described above, the frequency setting unit 120sets the driving frequency of the first converter 16 according to thetemperature detected by the temperature detection element 10 or 11 thatdetects the temperature at the sheet non-passing portion of therotational member 1, by which the heat generation distribution of therotational member 1 in the generatrix direction is adjusted. Then, thepower control unit 119 controls the output voltage of the secondconverter 15 according to the temperature detected by the temperaturedetection element 9 that detects the temperature at the sheet passingportion, by which the control for keeping the temperature at the sheetpassing portion constant is performed. The voltage of the capacitor 1113is different between when the ON duty ratio of the second converter 15is set to 80% and when the ON duty ratio of the second converter 15 isset to 50%, as shown in FIG. 11. The heat generation amount of therotational member 1 is adjusted with use of this difference in thevoltage.

In the manner as described above, according to the present exemplaryembodiment, the driving frequency of the first converter 16 is setaccording to the temperature at the sheet non-passing portion of therotational member 1, and controls the output voltage of the secondconverter 15 according to the temperature at the sheet passing portionof the rotational member 1. This allows the temperature at the sheetpassing portion to be maintained at the control target temperature whilethe heat generation distribution of the rotational member 1 is keptadjusted to the heat generation distribution according to the size ofthe recording material P.

As will be described in detail in the second exemplary embodiment, thedriving frequency of the first converter 16 may be set according to thesize of the recording material P, instead of being set according to thetemperature at the sheet non-passing portion. The above-described effectcan be achieved by setting the driving frequency of the first converter16 according to at least either of the size of the recording material Pand the temperature at the sheet non-passing portion of the rotationalmember 1.

Further, an effective value of the voltage to be input into the firstconverter 16 may be adjusted by disposing a switching element, such as atriac, on an input side of the diode bridge 1102 without disposing thesecond converter 15, and performing phase control or wave number controlon this element. Further, the effective value of the voltage to be inputinto the first converter 16 may be adjusted by disposing a switchingelement, such as a field-effect transistor (FET) and an IGBT on anoutput side of the diode bridge 1102, and performing the phase controlor the wave number control on this element.

According to the present exemplary embodiment, the voltage is outputfrom the second converter 15 according to the ON duty ratio of theswitching element 1104 of the second converter 15 in a relationshipexpressed by the following expression;

output voltage=input voltage×ON duty ratio  (1).

Further, in the apparatus according to the present exemplary embodiment,a relationship as illustrated in FIG. 13 is established between an inputpower duty ratio of the power input into the first converter 16 and theON duty ratio of the switching element 1104 of the second converter 15.

The driving frequency of the first converter 16 is changed according toat least either of the size of the recording material P and thetemperature at the sheet non-passing portion of the rotational member 1as described above, but the change in the driving frequency of the firstconverter 16 leads to a change in input power supplied to the fixingunit A. For example, when images are successively printed on theplurality of recording material P, the temperature increases at thesheet non-passing portion, which may raise the necessity of changing thedriving frequency of the first converter 16 in the middle of thesuccessive printing in some cases. However, the change in the drivingfrequency in the middle of the successive printing leads to the changein the input power supplied to the fixing unit A, whereby thetemperature of the fixing unit A is destabilized and thus fixability ofthe image is affected.

FIG. 14A is a graph illustrating a relationship between the drivingfrequency of the first converter 16 and the input power supplied to thefixing unit A. This graph indicates the relationship when a constantvoltage is input into the first converter 16 (with the input power dutyratio set to 100%), and a horizontal axis and a vertical axis representthe driving frequency of the first converter and the input powersupplied to the fixing unit A, respectively. The characteristic lineillustrated in FIG. 14A can be derived from the following expression;

$\begin{matrix}{{P = \frac{V^{2}*R}{\sqrt{R^{2} + \left( {{2\; \pi \; f*L} - \frac{1}{2\; \pi \; f*C}} \right)^{2}}}},} & (2)\end{matrix}$

where P represents the input power supplied to the fixing unit A, Vrepresents the input power supplied to the first converter 16, Rrepresents a resistance value of the equivalent resistance of the fixingunit A (refer to FIG. 5), f represents the driving frequency of thefirst converter 16, L represents the equivalent inductance of the fixingunit A, and C represents the capacity of the resonance capacitor 1113.

As illustrated in FIG. 14A, it is revealed that the change in thedriving frequency of the first converter 16 leads to the change in theinput power to be supplied to the fixing unit A, even if the power inputinto the first converter 16 kept the same. As illustrated in FIG. 14B,power of 900 W×80%=720 W is input into the fixing unit A when thedriving frequency of the first converter 16 is set to 50 kHz and theinput power duty ratio of the second converter 15 is set to 80%. If thedriving frequency of the first converter 16 is switched to 32 kHzwithout changing the input power duty ratio from this state, the inputpower to be supplied to the fixing unit A increases to 1250 W×80%=1000W. Therefore, a measure for preventing or reducing the change in thepower input into the fixing unit A should be taken to prevent or reducethe change in the temperature of the fixing unit A (the change in thetemperature of the rotational member 1) when the heat generationdistribution is adjusted by changing the driving frequency of the firstconverter 16.

Therefore, the power control unit 119 according to the present exemplaryembodiment controls the second converter 15 according to the temperatureat the sheet passing portion of the rotational member 1 and the drivingfrequency of the first converter 16 to control the power to be suppliedfrom the second converter 15 to the first converter 16. Morespecifically, the power to be input into the fixing unit A is corrected(the input power duty ratio is corrected) based on the characteristicexpressed by the expression (2), when the driving frequency of the firstconverter 16 is switched. As illustrated in FIG. 13, according to thepresent exemplary embodiment, the input power (the input power dutyratio) is changed by changing the ON duty ratio of the second converter15. In the following description, a method for correcting the power tobe input into the fixing unit A will be described.

Assume that Ppre represents the input power when the first converter 16is driven with the driving frequency before the driving frequency ischanged. On the other hand, assume that Paf represents the input powerwhen the first converter 16 is driven with the driving frequency afterthe driving frequency is changed. Each of Ppre and Paf indicates theinput power of the corresponding driving frequency illustrated in FIG.14A. Assume that Dpre represents the input power duty ratio before thedriving frequency of the first converter 16 is changed, and Dafrepresents the input power duty ratio after the driving frequency ischanged, which is set in such a manner that the input power matches theinput power before the driving frequency of the first converter 16 ischanged. The input power duty ratio Daf after the driving frequency ischanged can be expressed by the following expression;

Daf=Ppre/Paf*Dpre  (3).

The engine control unit 121 recognizes Dpre, and can calculate Ppre andPaf from the expression (2). FIG. 14C illustrates an example in whichthe input power is also corrected when the driving frequency is changed.In the example illustrated in FIG. 14C, Dpre is 80%, and Daf can beobtained to be 58% by calculating Ppre of when the driving frequency is50 kHz and Paf of when the driving frequency is 32 kHz, and substitutingthem into the expression (3). Therefore, the input power supplied to thefixing unit A can be maintained at 720 W even when the driving frequencyof the first converter 16 is switched from 50 kHz to 32 kHz.

According to the present exemplary embodiment, Ppre and Paf arecalculated from the expression like the expression (2), but an inputpower table indicating input power for each driving frequency, asillustrated in FIG. 14A, may be prepared in the engine control unit 121in advance, and Daf may be acquired with use of the table.

FIG. 15 is a flowchart illustrating a power input sequence adopting thecorrection of the input power duty ratio according to the presentexemplary embodiment. In step S101, the driving frequency is temporarilyset to f1=50 kHz (hereinafter, f1 will be described as a drivingfrequency after the driving frequency is switched and changed) as aninitial setting 1. In step S102, the temporarily set driving frequencyf1=50 kHz is replaced with f0 (hereinafter, f0 will be described as adriving frequency before the driving frequency is switched and changed),as an initial setting 2. In step S103, the input power duty ratio Dpreof the power input from the second converter 15 is determined, the inputpower duty ratio Dpre of the power input from the second converter 15being of when the first converter 16 is driven with the drivingfrequency f0, which allows the temperature of the fixing unit A to bemaintained at the target temperature by the PID control or the like,from the temperature detected by the temperature detection element 9 andthe target temperature. As a result, it becomes possible to clarify thepower required to maintain the temperature of the fixing unit A at thetarget temperature when the driving frequency of the first converter 16is the driving frequency f0.

In step S104, the after-change driving frequency f1 is determinedaccording to the size of the recording material P or the temperaturedetected by the temperature detection element 10. Subsequently, in stepS105, whether the driving frequency of the first converter 16 should bechanged is determined by comparing the driving frequencies f0 and f1. Iff0#f1, so that the driving frequency should be changed (NO in stepS105), the processing proceeds to step S106. In step S106, Ppre and Pafare calculated from the expression (2). In step S107, the after-changeinput power duty ratio Daf is calculated. In steps S106 and S107, theinput power duty ratio Daf of when the driving frequency is switched tothe driving frequency f1 is calculated in such a manner that the inputpower matches the input power of when the input power duty ratio is setto the input power duty ratio Dpre calculated in step S103.

On the other hand, if f0=f1 as a result of the comparison in step S105(YES in step S105), it is determined not to change the drivingfrequency. Then, in step S108, Daf is replaced with Dpre. In step S109,inputting the power is started with the input power duty ratio of thesecond converter 15 set to Daf, and the driving frequency of the firstconverter 16 set to f1. The power control unit 119 drives the switchingelement 1104 of the second converter 15 according to the input powerduty ratio Daf to adjust the effective voltage to be input into thefirst converter 16. In the power input sequence according to the presentexemplary embodiment, the input power is updated according to thetemperature detected by the temperature detection element 9 for eachperiod of one cycle of the alternating-current waveform (a frequency of50 Hz or 60 Hz) of the commercial power source 50 (a control cycle or anupdate cycle). Further, whether the timing of updating the input powerhas come is determined by counting the number of half waves (=½ cycles)of the alternating-current waveform of the commercial power source 50with use of a counter t. In step S110, the counter t is reset. In a casewhere the counter t does not reach the control cycle (the cycle forupdating the duty ratio Daf) T in step S111 (NO in step S111), in stepS112, the counter t is incremented. In a case where the counter treaches or exceeds the control cycle T in step S111 (YES in step S111),and if inputting the power is continued in step S113 (NO in step S113),Daf and f1 is calculated for the next control cycle T, and inputting thepower is continued. In a case where inputting the power is stopped instep S113 (YES in step S113), the present sequence is ended. The controlcycle T of the power does not necessarily have to be one cycle of thealternating-current waveform of the commercial power source 50, and maybe two or more cycles.

In this manner, the input power duty ratio of the power input from thesecond converter 15 is changed from Dpre to Daf at the timing of whenthe driving frequency of the first converter 16 is switched. Thiscontrol allows the image forming apparatus 100 to prevent or reduce atemperature ripple, which would otherwise occur on the fixing unit A,while preventing or reducing the increase in the temperature at thesheet non-passing portion when the small-sized recording material P isprocessed by the fixing processing.

According to the first exemplary embodiment, the input power duty ratioDaf is calculated in such a manner that the input power after thedriving frequency is changed becomes substantially equal to the inputpower before the driving frequency is changed.

A configuration that sets the input power duty ratio Daf so as to keepthe input power constant regardless of the driving frequency will bedescribed as an exemplary modification of the first exemplaryembodiment. By this setting, the present exemplary modification allowsthe input power to be unchanged or less changed even when the drivingfrequency of the first converter 16 is changed. Assuming that a drivingfrequency corresponding to input power serving as a reference (a target)is a reference driving frequency fk, the present exemplary modificationcalculates the input power duty ratio Daf corresponding to the drivingfrequency f1, from the input power corresponding to the referencedriving frequency fk and the input power corresponding to the drivingfrequency f1 with which the first converter 16 is actually driven.

Assume that Pk represents the power corresponding to the referencedriving frequency fk, and P represents the power corresponding to thedriving frequency f1. Further, assume that Dk represents the input powerduty ratio corresponding to the reference driving frequency fk, and Dafrepresents the input power duty ratio corresponding to the drivingfrequency f1 that is calculated in such a manner that the power matchesthe power Pk. The input power duty ratio Daf can be expressed by thefollowing expression;

Daf=Pk/P*Dk  (4).

The power Pk and the power P can be calculated from the expression (2).

FIG. 16 is a flowchart illustrating a power input sequence adopting thecorrection of the input power duty ratio according to the presentexemplary modification. Descriptions of the points similar to the onedescribed with reference to FIGS. 14A to 14C will be omitted here. Aftera start of the power input sequence, in step S201, the reference drivingfrequency fk (40 kHz indicated in FIG. 14A in the present exemplarymodification) is set, which is an only initial setting in this sequence.In step S202, the input power duty ratio Dk of the power input from thesecond converter 15 is determined, the input power duty ratio Dk of thepower input from the second converter 15 being of when the firstconverter 16 is driven with the reference frequency fk, from thetemperature detected by the temperature detection element 9 and thetarget temperature. Subsequently, the driving frequency f1, and Pk and Pare calculated in a similar manner described in the flowchartillustrated in FIG. 15. In step S205, the input power duty ratio Daf isdetermined with use of the expression (4). The sequence after that issimilar to the one described with reference to FIGS. 14A to 14C.

The present exemplary modification sets the target power to thepredetermined power corresponding to the reference driving frequency fkregardless of the driving frequency f1, thereby allowing the input powersupplied to the fixing unit A to be unchanged or less changed and thetemperature ripple of the fixing unit A to be prevented from occurringeven when the driving frequency is changed.

According to the first exemplary embodiment and the exemplarymodification thereof, the input power duty ratio of the second converter15 is switched at the same time when the driving frequency of the firstconverter 16 is switched. However, the duty ratio may be switched at atiming before or after a predetermined time of when the drivingfrequency is changed. The predetermined time here can be severalmilliseconds or dozens of milliseconds.

According to the above-described first exemplary embodiment andexemplary modification thereof, the film-shaped member is used as therotational member (the fixing sleeve) 1. However, the present inventioncan be also applied to a fixing apparatus using a rigid rotationalmember having little flexibility as the rotational member with the coreand the coil disposed therein.

A second exemplary embodiment is configured to set the driving frequencyof the first converter 16 according to the size of the recordingmaterial P while the first exemplary embodiment is configured to set thedriving frequency of the first converter 16 according to the temperatureat the sheet non-passing portion.

As described above, the heat generation amount becomes lower at thelongitudinal ends of the rotational member 1 than at the central portionof the rotational member 1, as the driving frequency of the firstconverter 16 reduces. The present exemplary embodiment utilizes thischaracteristic, and sets the driving frequency of the first converter 16to a lower frequency as a width (a width in a direction perpendicular toa conveyance direction) of the recording material P reduces. Thefollowing table indicates the driving frequency for each size of therecording material P.

Letter Size A4 Size B5 Size A5 Size Size Of Width Width Width WidthRecording 216 mm 210 mm 182 mm 148 mm Material P Length Length LengthLength 279.4 mm 297 mm 257 mm 210 mm Driving 50 kHz 44 kHz 36 kHz 20 kHzFrequency

According to the present exemplary embodiment, the frequency settingunit 120 sets the driving frequency according to the information aboutthe size of the recording material P that is specified by a user via thehost computer 42.

The driving frequency used when the recording material P correspondingto one size is processed by the fixing processing may be alternatelyswitched between a first driving frequency and a second drivingfrequency lower than a first driving frequency. FIG. 12 illustratestemperature distributions of the rotational member 1 in a rotationalaxis direction in a case where a change is made in a ratio per unit timebetween a driving time period during which the first converter 16 isdriven with the driving frequency of 20 kHz, and a driving time periodduring which the first converter 16 is driven with the driving frequencyof 50 kHz. For example, assume that the ratio between the driving timeperiods as 20 kHz:50 kHz is 10:0 when the recording material P has theA5 size. Assume that the ratio between the driving time periods as 20kHz:50 kHz is 5:5 when the recording material P has the B5 size. Assumethat the ratio between the driving time periods as 20 kHz:50 kHz is 1:9when the recording material P has the A4 size. Assume that the ratiobetween the driving time periods as 20 kHz:50 kHz is 0:10 when therecording material P has the letter size.

According to the above-described first and second exemplary embodiments,the film-shaped member is used as the rotational member (the fixingsleeve) 1. However, the present invention can be also applied to thefixing apparatus using a rigid rotational member having littleflexibility as the rotational member with the core and the coil disposedtherein.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-148884, filed Jul. 22, 2014, and No. 2014-189087, filed Sep. 17,2014, which are hereby incorporated by reference herein in theirentirety.

What is claimed is:
 1. A fixing apparatus configured to fix an imageonto a recording material, the fixing apparatus comprising: a rotationalmember having a cylindrical shape and configured to include a conductivelayer; a coil having a helical shape and configured to be disposedinside the rotational member, the coil having a helix axis extending ina direction along a generatrix direction of the rotational member; aresonance circuit including a resonance capacitor and configured to beformed together with the rotational member and the coil; a firstconverter configured to drive the resonance circuit; a second converterconfigured to be used to control power to be supplied to the firstconverter; a frequency setting unit configured to set a drivingfrequency of the first converter according to at least one of a size ofthe recording material and a temperature at a sheet non-passing portionof the rotational member; and a power control unit configured to controlthe second converter according to a temperature at a sheet passingportion of the rotational member to control the power to be supplied tothe first converter from the second converter, wherein the conductivelayer is caused to generate heat by electromagnetic induction, and theimage formed on the recording material is fixed onto the recordingmaterial with the heat of the rotational member.
 2. The fixing apparatusaccording to claim 1, wherein the power control unit controls the secondconverter so as to maintain the temperature at the sheet passing portionof the rotational member at a target temperature.
 3. The fixingapparatus according to claim 1, wherein the resonance circuit is acurrent resonance circuit.
 4. The fixing apparatus according to claim 1,wherein the second converter is a voltage step-down converter.
 5. Thefixing apparatus according to claim 1, further comprising a coredisposed inside a helical shaped portion of the coil and configured toguide the magnetic flux, wherein an induced current flowing in acircumferential direction of the rotational member is generated in theconductive layer.
 6. The fixing apparatus according to claim 5, whereinthe core has a shape having an end.
 7. A fixing apparatus configured tofix an image onto a recording material, the fixing apparatus comprising:a rotational member having a cylindrical shape and configured to includea conductive layer; a coil having a helical shape and configured to bedisposed inside the rotational member, the coil having a helix axisextending in a direction along a generatrix direction of the rotationalmember; a resonance circuit including a resonance capacitor andconfigured to be formed together with the rotational member and thecoil; a first converter configured to drive the resonance circuit; asecond converter configured to be used to control power to be suppliedto the first converter; a frequency setting unit configured to set adriving frequency of the first converter according to at least one of asize of the recording material and a temperature at a sheet non-passingportion of the rotational member; and a power control unit configured tocontrol the second converter according to a temperature at a sheetpassing portion of the rotational member and the driving frequency tocontrol the power to be supplied to the first converter from the secondconverter, wherein the conductive layer is caused to generate heat byelectromagnetic induction, and the image formed on the recordingmaterial is fixed onto the recording material with the heat of therotational member.
 8. The fixing apparatus according to claim 7, whereinthe power control unit controls the second converter so as to maintainthe temperature at the sheet passing portion of the rotational member ata target temperature.
 9. The fixing apparatus according to claim 7,wherein the resonance circuit is a current resonance circuit.
 10. Thefixing apparatus according to claim 7, wherein the second converter is avoltage step-down converter.
 11. The fixing apparatus according to claim7, further comprising a core disposed inside a helical shaped portion ofthe coil and configured to guide the magnetic flux, wherein an inducedcurrent flowing in a circumferential direction of the rotational memberis generated in the conductive layer.
 12. The fixing apparatus accordingto claim 11, wherein the core has a shape having an end.